A cap of around 0.75% will be imposed next year on pension scheme charges applying to at least 10 million workers covered by new auto-enrolment schemes, government sources have said.

In what is likely to be seen as a populist pre-election move, the pensions minister Steve Webb is expected to announce the cap in April 2015, a month before the general election.

Confirmation of the cap came after Labour accused ministers of allowing the pension industry to "nobble" the reform plans. Gregg McClymont, the shadow pensions minister, said a "botched" consultation by Webb would deprive future pensioners of tens of thousands of pounds.

McClymont spoke out after Webb appeared to suggest in a written ministerial statement that a cap had been abandoned during this parliament because no change would be possible until April 2015.

Webb said: "We remain strongly minded to cap pension scheme charges in the default funds used for automatic enrolment. However, we have consistently encouraged firms to start getting ready for automatic enrolment 12 months ahead of the time the new employer duties apply to them.

"Therefore, to give those employers at least 12 months' notice of the rules that will apply to them, I can confirm that any cap on charges will not be introduced before April 2015."

It is understood that Webb is adamant that a cap will be imposed on auto-enrolment schemes from April next year. His statement to parliament was written with care because the government is obliged to give the pension providers 12 months' notice of any changes. Government sources said Webb would serve notice this April.

Webb gave a strong hint of the plans when he said: "I have seen nothing in the responses to the consultation that changes my view that effective action is needed on pension scheme charges."

Earlier the regulatory policy committee dismissed as "not fit for purpose" an impact assessment on the changes. McClymont told the Guardian: "The government's botched consultation has allowed vested interests to nobble the tough action required to deliver for Britain's savers."

He said the government had let down future pensioners. "As far as I understand it the pensions minister maintains that he is committed to a charge cap on pensions. But anyone who reads the written ministerial statement smuggled out today by the minister will see that there is no commitment to a charge cap on any timetable.

"In fact all there is the suggestion that any charge cap could not be introduced before April 2015. Given that by April 2015 we will be in the midst of one of the toughest-fought election campaigns in living memory it seems unlikely that this cap will ever be imposed in a meaningful fashion."

He added: "The minister promised, when he launched the charge cap consultation, a full-frontal assault on pension charges. What we have today is a full-scale retreat. This matters not just because the government has said one thing and now appears to be doing something else but because the millions of people who are being enrolled in a pension for the first time now will not have the protection that a properly designed cap could provide."

Labour believes it won the qualified support of an important ally this week when Lord Lawson, the former Tory chancellor, threw his weight behind calls for more transparent disclosure of pension scheme transaction costs. However, he added that proper disclosure would mean no need for a cap.

Lawson told peers: "What I have gone for is compulsory disclosure. In a competitive market, compulsory disclosure will go a very long way towards removing the mischief. If there is proper disclosure, there is no need for a cap or the regulation of charges in the first instance."

He rounded on a call by the Investment Management Association for it to be allowed to write a statement on cost disclosures for fund managers. He said: "This is ludicrous. You are asking the foxes to regulate the hen coop, as it were."